The fetus in utero bears paternal antigens and should be rejected as a foreign graft by the maternal immune system, an immunological enigma described by Peter Medawar more than 50 years ago. Maternal leukocytes that populate the uterine decidua certainly play an important role in tolerance. They interact with placental extravillous trophoblasts (EVT) that attach to and invade the decidua. EVT express an unusual Class I MHC protein of unknown function, the HLA-G dimer that is found only in pregnant women and apes. The goal of this proposal is to characterize both maternal leukocytes and fetal EVT and to study their interactions that must contribute to various aspects of pregnancy, including remodeling of the uterine spiral arteries to increase blood flow to the developing fetus, implantation of the blastocyst (16 cell stage embryo) into the uterus and maternal- fetal tolerance. Decidual natural killer cells (~70% of leukocytes in the decidua that were characterized during the first 5 years of this project) contribute to maternal-fetal tolerance by several means. This proposal seeks to characterize the remaining major sets of decidual leukocytes, the macrophages (~20% of the leukocytes) and T cells (5-20%), to describe synapses that occur between these cells, and to expand our knowledge of the interaction of HLA-G dimer bearing EVT with decidual leukocytes. Microarray analysis will be used as a principal tool in characterizing human decidual macrophages in order to establish whether they are M2 regulatory macrophages and what factors they may secrete. They may have an important role in maternal-fetal tolerance and in uterine remodeling. Studies of decidual T cells (5-20% of the leukocytes) that include regulatory T cells will also be initiated. The EVT that invade the decidua and intermingle with the decidual leukocytes have been recently isolated. They will be characterized further and used to study their interaction with decidual natural killer cells and macrophages. We have already established that these interactions lead to the secretion of several cytokines and hypothesize that other proteins involved in maternal- fetal tolerance and in uterine remodeling are also secreted. Finally, the mechanism by which LIF contributes to implantation will be examined by studying its effects on EVT that express the LIF receptor. LIF knockout female mice are infertile because the blastocyst formed after conception cannot implant into the uterus. This project is directly related to reproductive failure in women, i.e. recurrent spontaneous abortion (RSA), some forms of infertility due to failure of implantation of the blastocyst, preeclampsia and fetal growth restriction due to the failure of vascular remodeling that provides an adequate blood supply.